History of Fiber Optics

The idea of using light waves for communication can be traced as far back as 1880 when Alexander Graham Bell invented the “photo-phone” shortly after he invented the telephone in 1876. In this remarkable experiment, speech was transmitted by modulating a light beam, which traveled through air to the receiver. The flexible reflecting diaphragm (which could be activated by sound) was illuminated by sunlight. The reflected light was received by a parabolic reflector placed at a distance of about 200 m.
The parabolic reflector concentrated the light on a photo-conducting selenium cell, which formed a part of a circuit with a battery and a receiving earphone. Sound waves present in the vicinity of the diaphragm vibrated the diaphragm, which led to a consequent variation of the light reflected by the diaphragm. The variation of the light falling on the selenium cell changed the electrical conductivity of the cell, which in turn changed the current in the electrical circuit. This changing current reproduced the sound on the earphone.

Fiber Optic Technology

Fiber Optic is a technology that uses glass as thin as a human hair to transmit data from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. It is widely used for communications, which permit transmits data over longer distances and at higher bandwidths than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. It is specially designed for a variety of other applications, including sensors and fiber lasers.
The process of communicating using fiber-optics involves an optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.

Components of a Fiber Optic Cable

* Core - Thin glass center of the fiber where the light travels. * Cladding - Outer optical material surrounding the core that reflects the light back into the core. * Buffer Coating - Plastic coating that protects the fiber from damage and moisture. Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket.

How Does an Optical Fiber Transmit Light?4
Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.
The light in a fiber-optic cable travels through the core by constantly bouncing from the cladding, which are mirror like walls. This is what is known as total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances.
However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light * 850 nm = 60 to 75 percent per km * 1,300 nm = 50 to 60 percent per km * 1,550 nm is greater than 50 percent per km
Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

Types of Optical Fibers4

Single-Mode Fibers: have small cores about 10 microns in diameter and transmit infrared laser light of the wavelength is 1,300 to 1,550 nanometers. Single-mode fibers are used for long distance communications. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

www.photonics.com www.photonics.com Multi-Mode Fibers: have larger cores about 62.5 microns in diameter and transmit infrared light of wavelength is 850 to 1,300 nanometers from light-emitting diodes. Multi-mode fibers are used mostly for short distances, up to 550 meters.

A Fiber-Optic Relay System

To understand how optical fibers are used in communications systems, let's look at an example where two naval ships in a fleet need to communicate with each other while maintaining radio silence. One ship pulls up alongside the other and the captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code and uses a signal light to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain.
Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. Fiber-optic relay systems consist of the following: 1) Transmitter: The transmitter is like the sailor on the deck of the sending ship. It receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal. The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum). 2) Optical Regenerator: As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical regenerators are spliced along the cable to boost the degraded light signals. 3) Optical Fiber: An optical regenerator consists of optical fibers with a special coating, called doping. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal. 4) Optical Receiver: The optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.

An Easier Way to Understand Fiber Optic Transmissions

Over the last 20 years or so, fiber optic lines have taken over and transformed the long distance telephone industry. Optical fibers are also a huge part of making the Internet available around the world. When fiber replaces copper for long distance calls and Internet traffic, it dramatically lowers costs.
To understand how a fiber optic cable works, imagine an immensely long drinking straw or flexible plastic pipe. For example, imagine a pipe that is several miles long. Now imagine that the inside surface of the pipe has been coated with a perfect mirror. Now imagine that you are looking into one end of the pipe. Several miles away at the other end, a friend turns on a flashlight and shines it into the pipe. Because the interior of the pipe is a perfect mirror, the flashlight's light will reflect off the sides of the pipe and you will see it at the other end. If your friend were to turn the flashlight on and off in a morse code fashion, your friend could communicate with you through the pipe. This is exactly how a fiber optic cable passes on signals.
Making a cable out of a mirrored tube would work, but it would be bulky and it would also be hard to coat the interior of the tube with a perfect mirror. A real fiber optic cable is therefore made out of glass. The glass is incredibly pure so that, even though it is several miles long, light can still make it through. The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic.
By coating the glass in plastic, you get the equivalent of a mirror like environment around the glass strand. This mirror creates total internal refraction, just like a perfect mirror coating on the inside of a tube does. You can experience this sort of reflection with a flashlight and a window in a dark room. If you direct the flashlight through the window at a 90-degree angle, it passes straight through the glass. However, if you shine the flashlight at a very shallow angle (nearly parallel to the glass), the glass will act as a mirror and you will see the beam reflects off the window and hit the wall inside the room. Light traveling through the strands of fiber optic cable bounces at shallow angles in the same fashion and stays within the fiber without escaping.
To send telephone conversations through a fiber optic cable, analog voice signals are translated into digital signals. A laser or a light diode at one end of the pipe switches on and off to send each bit. Modern fiber systems with a single laser can transmit billions of bits per second. The laser within can actually turn on and off several billions of times per second. The newest systems use multiple lasers with different colors to fit multiple signals into the same fiber. This is known as single vs multi fiber cables.
Advantages:
1) Speed: Fiber optic networks operate at high speeds up into the gigabits than copper cables. 2) Bandwidth: large carrying capacity which means they can carry more data. 3) Distance: Signals can be transmitted further without needing to be refreshed or strengthened. 4) Resistance: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables. 5) Maintenance: Fiber optic cables costs much less to maintain. 6) Non-Flammable - Because no electricity is passed through optical fibers, there is no fire hazard. 7) Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes: i. Medical Imaging - in bronchoscopes, endoscopes, laparoscopes ii. Mechanical Imaging - inspecting mechanical welds in pipes and engines iii. Plumbing - to inspect sewer lines

Disadvantages: 1) Expensive: It is expensive to install compare to other cables. 2) Fragile: It is easy to break if it is not wrapped properly because it uses glasses, and it is also nearly impossible to splice.
Works Cited

1. Ghatak, Ajoy, and K. Thyagarajan. "Optical Waveguides and Fibers."Fundamentals of Photonics (n.d.): 249-92. Web. 5 Mar. 2016.

2. "Fiber Optic Components." Fiber Optic Components. N.p., 18 Aug. 2014. Web. 05 Mar. 2016.

3. Hogan, Hank. "Bandwidth Demands Drive Fiber Optics Advances."Photonics. N.p., n.d. Web. 5 Mar. 2016.

4. Craig Freudenrich, PH.D. "How Fiber Optics Work." HowStuffWorks. N.p., 06 Mar. 2001. Web. 3 Mar. 2016.

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[ 1 ]. Ghatak, Ajoy, and K. Thyagarajan
[ 2 ]. 4 Craig Freudenrich
[ 3 ]. 4 Craig Freudenrich